Epoxy resins are well established as a class of curable compositions which find efficacy in a myriad of applications. The curing of epoxy resins is effected by a wide range of curing agents including, for example, the primary and secondary aliphatic, cycloaliphatic and aromatic polyamines; dicarboxylic acids and the anhydrides thereof; aromatic hydroxyl containing compounds; imidazoles; guanidines; urea-aldehyde resins and alkoxylated derivatives thereof; melamine-aldehyde resins and alkoxylated derivatives thereof; amidoamines; epoxy resin adducts; and various combinations thereof. In addition to said curing agents, one or more catalysts, such as a quaternary ammonium or phosphonium salts are frequently added to accelerate the cure rate as well as to insure completeness of the cure. In the curing of epoxy resins with curing agents using catalysts, a number of factors are critical to and interrelate to the curing process for curable compositions containing mesogenic moieties relative to conventional, non-mesogenic curable compositions, directly as a result of the presence of the mesogenic moieties. These factors include the relatively higher processing temperatures required to melt or dissolve most mesogenic epoxy resins or mesogenic curing agents in the presence of a catalyst as well as the development and maintainance of the desired morphology under the curing time and temperature profile employed. Thus, attempting to cure a curable mixture of epoxy resin and a curing agent, one or both of which contain mesogenic moieties, in the presence of a catalyst under conditions required for melt processing of said curable mixture frequently results in premature crosslinking of the matrix due to the cure accelerating effect of the catalyst. The result of this premature crosslinking is higher viscosities, lack of control over the type of morphology attained in the cured state, underdevelopment of the liquid crystalline morphology and a subsequent deleterious impact on mechanical properties of the cured product.
The present invention provides novel curable mixtures comprising at least one epoxy resin and at least one curing agent therefor, and at least one cure controlling catalyst therefor; wherein, at least one of the epoxy resins or at least one of the curing agents contains at least one mesogenic moiety.
It has been discovered that in curable compositions containing mesogenic moieties, the use of a cure controlling catalyst allows higher processing temperatures to be used for melting or dissolving the mesogenic epoxy resin and/or mesogenic curing agent, and lower viscosities and greater control over the morphologies and properties are also obtained. Thus, for curable compositions containing mesogenic moieties of the present invention, lower levels of crosslinking at high temperatures prior to catalyst activation and improved processability have been achieved through the use of a cure controlling catalyst. Additionally, the fracture toughness achieved for curable compositions containing mesogenic moieties and a cure controlling catalyst has been found to be unexpectably higher than that obtained for curable compositions which contain non-mesogenic moieties.